A wideband spectropolarimetry study of the spatially resolved radio galaxies: Cygnus A & Hydra A
- Sebokolodi, Makhuduga Lerato Lydia
- Authors: Sebokolodi, Makhuduga Lerato Lydia
- Date: 2022-04-04
- Subjects: Radio astronomy , Radio galaxies , Faraday effect , Astrophysical spectropolarimetry , Intracluster medium , Cosmic magnetic fields
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/454415 , vital:75342 , DOI 10.21504/10962/454415
- Description: This study presented results from our deep, wideband, high-spectral and high-spatial-resolution polarisation observations of the two most powerful radio galaxies, namely Cygnus A and Hydra A, with the Jansky Very Large Array (JVLA). The high surface brightness and strong polarisation of these radio sources enabled detailed polarimetric imaging, providing images at 0.75′′ resolution across 2−18 GHz and 2000 independent lines-of-sight across Cygnus A, and images at 1.5′′ (2 − 12 GHz) and 600 lines-of-sight across Hydra A. Our data revealed significant depolarisation and depolarisation structure, as well as deviations from a _2-law. We also found complicated structures in the Faraday spectra ranging from single-peaked to blended/resolved double- and multiple-peaked. The Faraday spectra of Hydra A were more multiple-peaked than Cygnus A. The fractional polarisation increased monotonically with increasing resolution, as expected. However, there were numerous lines-of-sight with complicated behaviour. We also found that the structure and complexity in the depolarisation increased at lower resolutions, suggesting substantial spatial structures across the lobes/tails. We fitted the 0.3′′ (6−18 GHz) and 0.50′′ (6−12 GHz) images of Cygnus A and Hydra A, respectively, with a simple model incorporating random, unresolved fluctuations in the cluster magnetic field to determine the high-resolution, high-frequency properties of the sources and the cluster. We found rotation measures (RM) between −5000 rad m−2 and +6400 rad m−2 across Cygnus A, and −2000 rad m−2 and +11900 rad m−2 across Hydra A, consistent with previous studies. From these derived properties, we generated predicted polarisation images of the sources at lower frequencies (< 6 GHz), convolved to 0.75′′ for Cygnus A and 1.5′′ for Hydra A. The predictions were remarkably consistent with the observed emission in both sources, providing strong support for the depolarisation being a result of unresolved fluctuations in the magnetic fields. We fitted various analytical models to the wideband data. We found that the data for both sources were inconsistent with a wholly mixed gas of thermal and synchrotron gas, particularly for regions withRM > 1000 rad m−2. Instead, the data required a dominant Faraday rotating screen in the foreground of the radio sources. The wideband modelling also showed preference towards models with at least two or more unresolved Faraday rotating patches. Single depolarising models fail to describe the data. This implies the presence of more than one depolarising screen in the vicinity of these sources. The observations were consistent with the lower-frequency depolarisation due to unresolved fluctuations on scales ≳ 300−700 pc in the magnetic field or the electron density superposed on a partially ordered field component. Both the large-scale magnetic fields and unresolved magnetic field fluctuations are external to the radio emission. The magnetic fields around Cygnus A are located in the ambient cluster gas, the shocked gas in the boundary of the lobes or both, while the magnetic fields around Hydra A are most likely located in the ambient cluster gas. , Thesis (PhD) -- Faculty of Science, Physics, 2022
- Full Text:
- Date Issued: 2022-04-04
- Authors: Sebokolodi, Makhuduga Lerato Lydia
- Date: 2022-04-04
- Subjects: Radio astronomy , Radio galaxies , Faraday effect , Astrophysical spectropolarimetry , Intracluster medium , Cosmic magnetic fields
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/454415 , vital:75342 , DOI 10.21504/10962/454415
- Description: This study presented results from our deep, wideband, high-spectral and high-spatial-resolution polarisation observations of the two most powerful radio galaxies, namely Cygnus A and Hydra A, with the Jansky Very Large Array (JVLA). The high surface brightness and strong polarisation of these radio sources enabled detailed polarimetric imaging, providing images at 0.75′′ resolution across 2−18 GHz and 2000 independent lines-of-sight across Cygnus A, and images at 1.5′′ (2 − 12 GHz) and 600 lines-of-sight across Hydra A. Our data revealed significant depolarisation and depolarisation structure, as well as deviations from a _2-law. We also found complicated structures in the Faraday spectra ranging from single-peaked to blended/resolved double- and multiple-peaked. The Faraday spectra of Hydra A were more multiple-peaked than Cygnus A. The fractional polarisation increased monotonically with increasing resolution, as expected. However, there were numerous lines-of-sight with complicated behaviour. We also found that the structure and complexity in the depolarisation increased at lower resolutions, suggesting substantial spatial structures across the lobes/tails. We fitted the 0.3′′ (6−18 GHz) and 0.50′′ (6−12 GHz) images of Cygnus A and Hydra A, respectively, with a simple model incorporating random, unresolved fluctuations in the cluster magnetic field to determine the high-resolution, high-frequency properties of the sources and the cluster. We found rotation measures (RM) between −5000 rad m−2 and +6400 rad m−2 across Cygnus A, and −2000 rad m−2 and +11900 rad m−2 across Hydra A, consistent with previous studies. From these derived properties, we generated predicted polarisation images of the sources at lower frequencies (< 6 GHz), convolved to 0.75′′ for Cygnus A and 1.5′′ for Hydra A. The predictions were remarkably consistent with the observed emission in both sources, providing strong support for the depolarisation being a result of unresolved fluctuations in the magnetic fields. We fitted various analytical models to the wideband data. We found that the data for both sources were inconsistent with a wholly mixed gas of thermal and synchrotron gas, particularly for regions withRM > 1000 rad m−2. Instead, the data required a dominant Faraday rotating screen in the foreground of the radio sources. The wideband modelling also showed preference towards models with at least two or more unresolved Faraday rotating patches. Single depolarising models fail to describe the data. This implies the presence of more than one depolarising screen in the vicinity of these sources. The observations were consistent with the lower-frequency depolarisation due to unresolved fluctuations on scales ≳ 300−700 pc in the magnetic field or the electron density superposed on a partially ordered field component. Both the large-scale magnetic fields and unresolved magnetic field fluctuations are external to the radio emission. The magnetic fields around Cygnus A are located in the ambient cluster gas, the shocked gas in the boundary of the lobes or both, while the magnetic fields around Hydra A are most likely located in the ambient cluster gas. , Thesis (PhD) -- Faculty of Science, Physics, 2022
- Full Text:
- Date Issued: 2022-04-04
The construction of a swept-frequency polarimeter for observing decametric radiation from Jupiter
- Authors: Baker, Dirk E.
- Date: 1970-01
- Subjects: Polariscope , Jupiter (Planet) Radiation , Faraday effect
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/447507 , vital:74652
- Description: This thesis describes the design and construction of a swept-frequency polarimeter for observing Jupiter’s decametric radiation in the frequency range from 15 to 45 MHz. The frequency band is tuned in two parts using two receivers for observing the left- and right-circular components of the radiation in the 15 to 26 MHz range and two receivers for observing the circular components in the 30 to 45 MHz range. The receivers are tuned electronically by means of varactor diodes and are of novel design in that they do not have any intermediate-frequency stages. The frequency band can be swept 10 times per second or 100 times per second. The antennas used for the polarimeter are helical beam antennas, two for each of the circular components in the 15 to 26 MHz range and two for the 30 to 45 MHz range. Model studies of the antennas were conducted to establish whether they have suitable characteristics for observing the polarisation of Jupiter’s decametric radiation over a wide frequency range. Many spectral records of Jupiter’s decametric radiation were obtained using a preliminary version of the swept-frequency receiver and a log-periodic dipole antenna. Some of the records are presented and one record which shows an interesting case of Faraday rotation is analysed. , Thesis (MSc) -- Faculty of Science, Physics, 1970
- Full Text:
- Date Issued: 1970-01
- Authors: Baker, Dirk E.
- Date: 1970-01
- Subjects: Polariscope , Jupiter (Planet) Radiation , Faraday effect
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/447507 , vital:74652
- Description: This thesis describes the design and construction of a swept-frequency polarimeter for observing Jupiter’s decametric radiation in the frequency range from 15 to 45 MHz. The frequency band is tuned in two parts using two receivers for observing the left- and right-circular components of the radiation in the 15 to 26 MHz range and two receivers for observing the circular components in the 30 to 45 MHz range. The receivers are tuned electronically by means of varactor diodes and are of novel design in that they do not have any intermediate-frequency stages. The frequency band can be swept 10 times per second or 100 times per second. The antennas used for the polarimeter are helical beam antennas, two for each of the circular components in the 15 to 26 MHz range and two for the 30 to 45 MHz range. Model studies of the antennas were conducted to establish whether they have suitable characteristics for observing the polarisation of Jupiter’s decametric radiation over a wide frequency range. Many spectral records of Jupiter’s decametric radiation were obtained using a preliminary version of the swept-frequency receiver and a log-periodic dipole antenna. Some of the records are presented and one record which shows an interesting case of Faraday rotation is analysed. , Thesis (MSc) -- Faculty of Science, Physics, 1970
- Full Text:
- Date Issued: 1970-01
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